1. Field of the Invention
The invention relates generally to an underwater propulsor, in particular to an oscillating-foil type underwater propulsor with a joint. The underwater propulsor of the invention mimics the tail movement of fast moving cetaceans or tunas and provides an oscillating foil and a joint for achieving better propulsion efficiency.
2. Description of Related Art
Regarding conventional underwater propulsors, there are a variety of applications such as flippers for diving or snorkeling, paddles for canoeing or Chinese dragon boats, and propellers of container ships or ocean liners. As an auxiliary equipment to facilitate propulsion, the flippers have long been important in the history of human underwater activities. Such a flipper has an appearance resembling the webbed toes of a frog, but it actually functions like a tail fin of slow moving fish. For a flat structure such as the flipper, the flat structure can easily cause early flow separation and stall when used as a propulsor in a fluid medium because it does not have a thicker foil thickness and a larger curvature radius of the leading edge of a high performance streamline foil, making it an inferior device for providing lift and taking load. Besides, the flipper also is a soft or non-rigid structure, which is prone to deformation and unable to take or transmit a larger thrust. Therefore, the flipper produces a very limited thrust in practical use.
The main propulsion method used in ships today is by the use of propellers. The marine main engine transfers the kinetic power through a shaft to the propeller, the blades of which then rotate to push the water backwards to generate a forward thrust, so that the ship is propelled. During the rotation of the fins, however, water surrounding the fins travels in a circumferential direction, which is almost helpless in facilitating the propulsion of the ship and becomes energy loss. For a conventional screw propeller, this problem has been a major barrier to further significant improvement of propulsion efficiency.
Furthermore, the screw propeller has another disadvantage, which is low maneuverability. When the mechanical power of the marine main engine is transferred to the propeller through the shaft fixed to the hull of the ship, the direction of the thrust produced by the propeller may not be controlled due to the fixed-positioned shaft. Therefore, the propeller by itself can only produce forward and backward thrusts but not side thrusts. A rudder is usually provided to change the direction of the water passing through the propeller to produce the side thrust needed. However, the rudder is only functional when the ship is going forward or backward, and that the side thrust is only a small fraction of the total thrust produced by the propeller, thus the ship can not be flexibly handled to move in the desired direction. To enhance the ship's maneuverability, a podded propulsor has been invented and developed for various applications in recent years. The podded propulsor resembles a small-scale submarine and has an advantage of being capable of rotating 360 degrees to overcome the steering problem. However, the propulsion method of the podded propulsor is the same as that of the propeller in that both types of propulsors use rotating blades and thus cause rotational kinetic energy loss. The propulsion method therefore has to be improved in order to enhance propulsion efficiency.
To solve the problems above, the invention provides a general-use underwater propulsor for various purposes.